I collect factory literature for the '64-'66 Chevy truck line. Per the booklet titiled "'64 Custom Features Accessories-For All Chevrolet Trucks", the referenced "cutout steps" are called "Pick-up Side Steps". They are described as "Permits easy access to the cargo in the front load space of the pick-up box".
They were custom feature accessories for the pick-up and panel trucks. Some of the other custom feature accessories also shown in this booklet, includes custom manual radio and antenna, seat belts, Jr. West Coast mirrors, windshield washer, power brakes, extendable rear view mirror, auxillary rear springs, front bumper guards, commercial grille guard, back-up lamps, locking gasoline cap, direction signal conversion, matched horn, hazard flasher switch, arm rest, cigarette lighter & lamp, electric clock, right hand sun visor, safety light and hand portable spot lamp.
I have found this literature and other pieces I have acquired, specifically for the '64-'66 Chevy trucks, has been very informative. It also makes it rather easy to determine what is factory/dealer accessories and what is a previous owner's bright idea.
